A Limnological Guide to Productivity Indicators: This guide outlines the limnological metrics and principles that we have used this year and discusses them in relationship to the productivity of a body of water. The following concepts/metrics are things you should be familiar with and will be expected to utilize in your discussion of the observations you make and data you collect during the process of your field final. You should be intimately familiar with the idea of trophic state. Excellent discussions of the concept can be found here: http://wow.nrri.umn.edu/wow/under/primer/index.html and here: http://dipin.kent.edu/trophic_state.htm Concepts to be familiar with: Morphological and Lake “setting” ideas:
Watershed Area to Lake Surface Area Ratio: There are numerous factors to consider here that modify the basic ratio (see below), but the basic idea here is that a lake with a large watershed compared to its own surface area (i.e., a large Aw:AL) will be more productive than a similarly sized lake with a smaller watershed. The idea here is that the larger the watershed the greater the potential nutrient loading from that watershed is. Of course if you have a very large lake, then it can dilute even very large nutrient loads…this is why the ratio is important and why just the size of the watershed taken by itself is inadequate to express the potential effect on water quality/productivity. In general, lakes with ratios < 5:1 are not particularly productive, between 5 and 10:1 moderately productive, and >10:1 more highly productive. But you do start to reach a limit…increases in productivity are not linear, a lake with a ratio of 100:1 would not be 10 times more productive than one with a 10:1 ratio, and might not be any more productive than a lake with a 30:1 ratio.
Watershed Landuse An essential modifier of the comparative area ratio discussed above. You are all familiar with the basics of this concept. It should be obvious to you by now that a smallish watershed that is dominated by agriculture can result in a higher nutrient load to a body of water than a large watershed dominated by forest. Thus, consideration of watershed landuse is key to understanding water quality/productivity.
Lake Volume and Flushing Rate all things being equal, a lake with a large volume can “absorb” a larger nutrient load than a small one because of the dilution factor. The flushing rate is also important in that large populations of phytoplankton don’t have the time to grow before being flushed out of the system if the flushing rate is very high. However, lakes with high flushing rates have to have the water coming from somewhere, and thus they may also have high nutrient loads to contend with.
filename: A Limnological Guide to Productivity Indicators.doc
Page 1 of 6
Watershed Geology You should be able to read the basic information from a geologic map. (You may have to do a little additional digging then to gain information about the chemical composition, hardness, and weathering characteristics of the particular rock type(s) involved). However, the basic thing we are most often concerned with is the extent to which the rocks contribute carbonates to the water supply. Carbonates are important to us for four reasons: 1. they form the “backbone” of the acid neutralizing capacity of the water. If the water is “well buffered” there will be little pH change in the system regardless of input, and thus the system is not likely to suffer from limited productivity due to high (or more likely) low pH’s. 2. The input of CO32- from dissolving limestone/marble (as CaCO3) in the watershed drives the equilibrium of the carbonate buffer system in the direction of CO2 improving the overall availability of CO2 for photosynthesis. 3. High levels of alkalinity (the concentration of the HCO3- and the CO32- ions) keep pHs on the high side of neutral. These basic (or “alkaline”) pH’s support chemical reaction conditions that result in the regeneration of phosphorus from sediments…thus high alkalinity can improve phosphorus availability. 4. the bicarbonate ion itself (HCO3-) can substitute for CO2 as a carbon source for some photosynthetic organisms, thus aquatic ecosystems with a good amount of alkalinity will tend to be more productive that a system with lower levels with all else being equal.
Factors we measure/collect data on:
Temperature: This may be one of the simplest things we measure but its importance can’t be overestimated. All aquatic life (i.e., phytoplankton, zooplankton, fishes, insects, etc…with the exception of aquatic mammals) are what are known as thermoconformers – their body temperature is equal to the temperature of the ambient environment. Since rates of chemical reactions are proportional to temperature, the rates of the chemical reactions essential to life are largely controlled by the temperature of the surrounding environment where thermoconformers are concerned. Because of this, aquatic organisms do everything more slowly when the temperature is colder. Thus, in comparing two lakes where every thing else is held equal, a warm lake will be more productive than a cold lake. You can extend this to think about the same lake at different times of the year, again, holding everything else constant, a lake should be more productive in the summertime than in the late fall or early spring based on temperature differences alone!
Oxygen saturation: (see this website for a nice discussion of oxygen saturation) http://www.esr.pdx.edu/pub/biology/limnology/limn-11.htm
We always look at dissolved oxygen concentrations (in mg/L) and think about how that may be affecting what kinds of organisms might live where in the volume of water, but we don’t always talk about the oxygen saturation value, a quantity which we measure in percent. You can quantify the amount of a gas (e.g., oxygen) that can be dissolved in water at a given temperature and pressure (this relationship is called Henry’s law). The
filename: A Limnological Guide to Productivity Indicators.doc
Page 2 of 6
colder the water is, the more gas that can be dissolved in it at any given pressure, and vice versa. This is why a warm soda or seltzer bottle is more likely to spray its contents all over you than one taken right out of the cooler. So, what’s the point here? With all else being equal, water that is just sitting around, should be in equilibrium with the atmospheric concentration of oxygen, and thus the saturation is 100% (i.e., the water is holding as great of a concentration of oxygen as is present in the overlying air at the given temperature and pressure). So ultimately what we are interested in is what causes deviations from 100%? What can make the water “supersaturated” and/or what can make it “undersaturated”? Think about it…what process produces oxygen? What processes consume it?…see below.
diel cycles ahhh yes, if there wasn’t a confusing term for it, it wouldn’t be science now would it? Diel just means daily…i.e., a 24 hour period. So a diel cycle is a phenomenon that repeats on a 24 hour time frame. In this context we are talking about oxygen saturation…so how might that change over a 24 hour period. Photosynthesis [PS] occurs only in the daylight, so increases in oxygen saturation should only increase during daylight hours. Cellular respiration [R] occurs 24hours a day…it is always consuming oxygen. So changes in oxygen saturation relate to the difference between how much is being produced minus the amount being consumed. So, as long as PS – R is > 0 the “DO” level is increasing and may exceed 100% saturation. If PS – R is < 0 the “DO” level is decreasing and may drop below 100% saturation. How does this relate to productivity? If the difference between the maximum saturation and minimum saturation in a 24 hour period is very large (e.g., from 140% down to 65%) then the lake is very productive (i.e., lots of photosynthesis, lots of respiration). If that difference is small (e.g., from 105% down to 88%) then the lake is not very productive. Think about the “Light Bottle/Dark Bottle” Lab and how you can calculate rates of O2 production and consumption and relate those back to the amount of biomass or energy that exists at the base of the trophic pyramid.
hypolimnetic oxygen deficits if a body of water thermally stratifies, then a cold hypolimnion forms that is essentially “decoupled” or disconnected from the water above in terms of any thermal or wind induced mixing. Thus, the oxygen concentration that was present there prior to stratification constitutes the entire pool of oxygen available to organisms in the hypolimnion until the lake mixes or “turns over” again in the fall. If the lake is highly productive there is a constant “rain” of organic material out of the epilimnion into the hypolimnon (e.g., dead phyto and zooplankton cells, zooplankton and fish fecal matter, etc, etc..). This provides a rich energy source for decomposers (mostly bacterial) in the benthos. As those decomposers “burn” the organic matter in the process of cellular respiration they consume the oxygen. This process of decomposition continually draws down the oxygen supply in the hypolimnion, until the oxygen runs out (and then other organisms with other types of metabolism take over). One of the critical implications of this absence of oxygen at/near the watersediment interfaces is that the chemical conditions for the regeneration of phosphate from the sediments becomes possible. So, in lakes where the near sediment waters are depleted of oxygen for a period of time (i.e., during summer stratification) those lakes are essentially “feeding” themselves
filename: A Limnological Guide to Productivity Indicators.doc
Page 3 of 6
phosphorous regardless of what watershed based phosphorous loading reduction strategies are being pursued. While that phosphorous is not available for phytoplankton growth during the warm, well-lit summer months (remember, it is “trapped” in the hypolimnion), it will get mixed up into the lake’s volume with fall turnover, thereby feeding a (light and temperature limited) fall algal “bloom” and much of it will remain available into the coming spring (depending on flushing rate).
Long story short: a highly productive lake that thermally stratifys will have an anoxic hypolimnion; lakes that aren’t as productive will maintain a fairly high/constant oxygen saturation in their hypolimnion throughout the year. Lakes that are highly productive and thermally stratify will tend to “regenerate” phosphorous from the sediments due to the anoxic conditions there. Essentially, these lakes keep “feeding” themselves. The relationship between phosphorous, high productivity, and hypolimnetic anoxia is a nasty positive feedback cycle. The trick to unlocking all of this is figuring out whether or not a lake stratifies or not.
Conductivity Conductivity is a “proxy variable.” A proxy is something that stands in for something else; in this case conductivity is a proxy for the load/concentration of Total Dissolved Solids (TDS) in the lake. We don’t try to measure TDS because it is such a cumbersome process involving careful weighing and drying of filters and the time consuming evaporation of all water out of the sample. The question is why are we interested in what is dissolved in the water? Primary producers (e.g., phytoplankton) can’t utilize any nutrient unless it is in solution (that is, dissolved in the water). Thus, measuring conductivity gives us an indication of the concentration of materials dissolved in the water and by extension an indicator of the potential primary productivity of the waterbody (and by further extension the productivity at higher trophic levels as well). Conductivity Total Dissolved Solids primary productivity secondary productivity
“TROPHIC STATE” Conductivity is cool because it is so darn easy to measure… but, you have to be careful how you use it because it is so general – you don’t know if conductivity is high because of a huge load of phosphorous in the system (which would likely make the system very productive) or because of a lot of road salt that has washed into the lake (which might limit productivity somewhat). So use conductivity carefully with an eye toward water sources and landuse. You should also realize that telling somebody that “the conductivity of a body of water is 200 µS” is next to meaningless if that person doesn’t have much limnological experience. You have to translate your numbers into a meaningful observation. You can do this by explaining what conductivity is, and by “scaling” it: high/medium/low. As in: “The conductivity of the body of water is quite low” It also helps to compare it to other lakes/bodies of water that people may have experience with: “The conductivity of X lake was 200 uS which compares with the very clear Riga Lake (15uS) and the algae-laden Hockey
filename: A Limnological Guide to Productivity Indicators.doc
Page 4 of 6
Pond (380uS)” That way you have created a comparative scale that helps me understand that 200 uS isn’t that high, but it is closer to a pond that is highly productive than to one which is not.
pH Acid Neutralizing Capacity CO2 production (H2CO3 formation) pH has the potential to be profoundly limiting to productivity. Most living things can only thrive in a fairly narrow range of pH’s (roughly between 6 and 8). While organisms can tolerate lower and higher pH’s their health and reproduction is generally negatively impacted by pH “extremes.” The problem here is that the enzymes necessary for managing an organism’s biochemistry tend to start to malfunction outside of the optimal pH range. Furthermore, low pH’s may result in chemical changes in the system that bring some metals (e.g., aluminum) into solution that can be toxic at even fairly low concentrations.
We also measure Alkalinity which is a measurement of the water’s “Acid Buffering Capacity” i.e., its ability to resist changes in pH. Most of the water around here is “well buffered” (alkalinities > 100 meq/L) and thus pH changes aren’t really a problem. However, these highly buffered lakes are actually fairly rare in Connecticut – it is a result of the rare geology of our region. The lower alkalinities of the Riga Lake are actually more typical. See section on Watershed Geology for related discussion.
The flip side of the diel cycling of oxygen is the continual production of CO2 as organisms consume the organic material produced by photosynthesis. (Everybody should be familiar with and recognize the implicit relationship between the two processes, understanding that photosynthesis is the process that ultimately supports all life, but that it is cellular respiration that “unlocks” and recovers the energy stored in the photosynthetic products for use by organisms). In any case, photosynthesis consumes CO2 and cellular respiration produces it. Given the fact that CO2 dissolves in water (albeit poorly) to produce the weak acid H2CO3 you should be able to draw a connection between CO2 and the pH of the water. The basic relationship is this: As photosynthesis consumes CO2 the pH of the system tends to rise, as cellular respiration produces CO2 pH tends to drop. While there are other factors involved, we can generally say that when photosynthesis dominates (during the day and only in the upper, well lit portions of a lake) pH will rise. When respiration dominates (during the nighttime (though note that respiration is a continuous process, occurring 24 hours a day) pH will fall.
Primary Producers This is where it all comes together. When you are talking about trophic state, you inevitably have to get around to a discussion of the actual organisms that are at the core of all of the biology going on in the system…the photosynthetic organisms that take inorganic materials from their environment and energy from the sun and work the alchemy that all other living things depend on. So, given that introduction, how do we go about quantifying the number of producers in a lake ecosystem? (Note that we are not talking about plants/macrophytes here, which are certainly important components of the ecosystem, but rather are focussing on phytoplankton which play a more important “food web” role than rooted plants do in most lakes of moderate or greater depth.)
filename: A Limnological Guide to Productivity Indicators.doc
Page 5 of 6
So, given that, here are three estimators of the biomass of primary producers in order of increasing resolution:
secchi transparency ahhh, the old standby…invented in 1866 by Angelo Secchi it is still the quickest and simplest and fairly reliable method of measuring the transparency of a volume of water. While there are a number of factors that may cause water to lose transparency (staining with tannic acids, suspended sediments), in most of the instances we encounter around here, transparency is most strongly affected by plankton in the water column. And because the physics of light are such that small particles affect light scattering more strongly than larger particles we will see the smaller phytoplankton affecting clarity more profoundly than the larger zooplankton do. All of this is a way of saying that all else being equal changes in secchi transparency are most strongly related to changes in the phytoplankton population. Thus, low sechhi transparencies are indicative of periods of high productivity, high secchi transparencies indicate periods of low productivity (or high grazing pressure).
chlorophyll a concentration Whooo…is it a pain to have to actually count phytoplankton…think about it, those suckers are small and there are a lot of them. It is a lot easier to use a proxy variable. Since all photosynthetic organisms have pigments to capture sunlight (e.g., chlorophyll) if you can measure the concentration of chlorophyll in a water sample it should be proportional to the amount of phytoplankton in that sample. Thus, we can extract and quantify chlorophyll from a known volume of water and use it to comment on productivity. High chlorophyll concentrations = high productivity, Low concentrations = low productivity. Do a simple search for “Chlorophyll concentration and Trophic State” on the internet to get info on how specific concentration levels equate with particular trophic states…and remember: you have collected your samples in Novemember, not June!
enumeration (and phytoplankton to zooplankton ratios) Sad to say, sometimes there just isn’t any substitute for actually counting the critters. This gives the most accurate picture of how many of what kind of things are in the water. By doing this count, you may also be able to identify some specific indicator species that are found only in certain types of conditions (e.g., high phosphorous/low nitrogen, or low alkalinity, or high silica, etc…). Furthermore, if you repeat this sampling over time you can get a sense of changing ratios between phytoplankton and zooplankton and get a sense of what the grazing pressure on the phytoplankton is like. Is the lake likely to become clearer or is it going to become more turbid? It depends on what part of the population cycle that you are looking at.
filename: A Limnological Guide to Productivity Indicators.doc
Page 6 of 6